1. Field of the Invention
The present invention relates to steam generators utilized in secondary and tertiary recovery of petroleum products and, more particularly, to a novel, compact steam generator such as can be utilized in conventional well casings or other confined areas.
2. The Prior Art
Petroleum, or crude oil, is found throughout the world and is generally located in subterranean pools formed in porous rock formations, such as sandstone. The liquid petroleum fills the voids between the various particles of sand or other rock that makes up the formation.
The physical characteristics of petroleum vary greatly depending upon the particular compounds which make up the petroleum deposit. Some petroleums are quite light and have a low viscosity such that they can easily flow through the formation and be pumped from the ground. Other petroleum reserves are much more viscous and are difficult to pump at ambient temperatures. Thus, the physical characteristics of the petroleum effect the ease with which the petroleum can be recovered.
Petroleum deposits are generally accompanied by deposits of natural gas. This natural gas can either be dissolved within the petroleum or it can form a small pocket on top of the petroleum pool. When an oil well is drilled, the gas pressure within the deposit is often times sufficient to force the petroleum to flow into the production well and up to the surface where it is recovered. As the gas pressure decreases through either expansion or recovery of the gas itself, it is often necessary to add pumps to the oil wells to assist in bringing the oil to the surface.
The utilization of the naturally existing gas pressures and the utilization of pumps are generally referred to collectively as primary recovery of oil. When pumping fails to provide sufficient stimulus to remove the oil from the ground, secondary and tertiary methods of oil recovery are often utilized.
One method of secondary recovery involves reinjecting natural gas into the formation to supply pressure to drive the oil up through the well. Often times, the oil remaining within the formation, after primary recovery is completed, has a high viscosity such that the mere addition of pressure is insufficient to cause it to flow to the well where it can be recovered. Accordingly, various methods of secondary recovery including those utilizing thermal stimulation have been devised to reduce the viscosity of the oil and enhance its ability to flow. Of the various thermal techniques which have been developed, steam injection has generally been demonstrated to be the most economical and most widely used.
According to the presently accepted techniques used in the industry, steam is generated above ground and is injected into the well casing where it is forced down into the petroleum-containing formation; the steam can then heat the petroleum to lower its viscosity. Although this process is widely utilized, it has several distinct disadvantages.
First, when the steam is injected into the well casing at the surface, enormous amounts of heat are lost as the steam passes down through the casing to the oil-bearing formation; this is often aggravated because many petroleum bearing formations are located thousands of feet below the surface of the ground. Thus, the effectiveness of the steam in reducing the viscosity of the oil is substantially lessened.
Second, depending on the process used for steam generation, heat losses to hot stack gases may be large, thus decreasing the overall thermal efficiency of the process.
Third, with all steam generators, it is necessary to burn some type of fuel to provide the necessary heat to creat the steam. Most fuels, unless they are thoroughly refined, contain compounds which create pollutants and thus, surface steam generators must be equipped with emission control devices which are costly and add to the expense of the operation.
Because of the disadvantages associated with above ground steam generators, many attempts have been undertaken to develop a down-hole steam generator. With down-hole generators, the steam is produced in the well near the formation where it is to be used, thereby reducing the amount of heat which is lost to surrounding formations and thereby improving the efficiency of the system. A second advantage of down-hole steam generators is that the noxious combustion products are generally discharged into the oil bearing formation where many of them are absorbed by the surrounding rock or become entrapped in the formation. Thus, the pollution problem is lessened to a great extent. Additionally, the combustion products formed during the creation of the steam contain a significant amount of carbon dioxide which is ultimately injected into the oil bearing formation where it is absorbed by the petroleum. Carbon dioxide injection is currently an accepted industrial process for tertiary recovery of hydrocarbons.
In spite of the many advantages associated with down-hole steam generators, attempts to develop a successfully operating unit have been plagued with various problems. For example, many of the prior art devices have experienced flame instability where pressure fluctuations in the well or supply tubes cause the flame to extinguish.
Additionally, it has been found difficult to maintain the thermal integrity of the burner components of the prior art devices during use. The flame temperature is generally high enough that it can melt or crack the walls of the generator if some type of cooling is not supplied. Thus, if the water flow rate decreases, is uneven, or if the lines or valves become plugged, the flame can easily damage the burner components. If a problem develops with the down-hole steam generator, it is necessary to pull the entire unit from the well casing which, in deep wells, causes considerable time delays and significantly increases the expense of the operation.
Finally, many of the prior art devices have experienced problems because of the unreliability of the ignition systems. Thus, fuel, oxidant, and water are pumped into the hole without any steam being generated.
In view of the foregoing, it will be appreciated that what is needed in the art is a down-hole steam generator and method which has a reliable ignition which produces a compact, high-intensity flame, which has a reliable cooling system, and which can operate for substantial periods of time without having to be removed from the well for repair or replacement of worn or destroyed parts. Such an apparatus and method are disclosed and claimed herein.